<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        
        let v1;//一个无意义的值 仍占据内存
        let v2 = null;//内存会腾出来 二者不是一回事
        console.log(v1);//undefined
        console.log(v2);//null
    </script>
</body>

</html>